The overall objective is to delineate the nature and extent of control exerted by major thermoreceptor areas in effecting the changes in heart rate (HR), iliac artery blood flow (IBF), and superior mesenteric artery blood flow (SMBF) that are produced during external heating of unanesthetized chaired baboons. These experiments will seek specifically to determine the role of skin temperature (Ts) and core temperature (Tc) changes in effecting the changes in IBF, SMBF, and HR during heat stress. Independent control of Tc will be obtained by use of a heat exchanger incorporated into a chronic femoral arteriovenous shunt, whereby Tc can be kept at control level during Ts elevation (external heating) or Tc can be elevated independent of Ts changes at both cool and elevated Ts levels. By these combinations of Tc and Ts, the relative importance of Ts vs. Tc in effecting the observed changes in IBF, SMBF, and HR, the nature of the Ts-measured variable and Tc-measured variable relationships (linear or otherwise), and the influence of Ts changes on Tc-measured variable relationship (i.e., altered slope and/or set point?) can be determined. Also, by use of appropriate cardiovascular drugs, the role of the sympathetic nervous system in producing the fall in SMBF during heat stress will be sought.